This invention relates to a magnetic recording medium, and more specifically to such a medium formed with a top coat in the form of a thin film of a polymer having the siloxane linkage to attain low friction, running stability, durability, corrosion resistance, and other desirable properties.
Considerable time has passed since magnetic recording media were first introduced consisting of a nonmagnetic base formed with a magnetic layer chiefly of a binder and a magnetic powder of an oxide, such as .gamma.-Fe.sub.2 O.sub.3, .gamma.-Fe.sub.3 O.sub.4, or Co-doped .gamma.-Fe.sub.2 O.sub.3. Recently, media for higher density recording with a coat composed of a binder and a ferromagnetic powder, such as Fe, Co, Ni, Fe--Co, Co--Ni, Fe--Co--Ni, Fe--Co--B, Fe--Co--Cr--B, Mn--Bi, Mn--Al, or Fe--Co--V, have come into use. More recently, those having a magnetic coat of either an evaporated metal deposit or a film formed by the sputtering technique are attracting increasing attention.
The magnetic recording media, above all those used as magnetic tapes and discs, are required to be low in friction coefficient, capable of running smoothly and stably, resistant to wear, stable in performance over an extended period of service, changeable little in properties, and accurately reproducible at any time under given ambient conditions. They are required to have durability and long life.
Aside from these requirements, surface leveling is under study for the magnetic recording media that use a feromagnetic layer for high density recording or have a film formed by vacuum evaporation or sputtering. Because of a tendency toward higher friction coefficients, some countermeasures have to be taken to ensure smooth, stable runs of the media. Another problem with those media is the possibility of deterioration due to corrosion of the metal particles exposed from the coats. Thus, there is particularly strong demand for magnetic recording media with greater running stability, smoothness, and durability than known heretofore so that the information recorded in high density can be stored for long periods and reproduced reliably and faithfully at any time desired.
With those media it has been customary to use silicone oil or other lubricant by either mixing it, with kneading, in the magnetic coating material or by applying it over the magnetic layer. Top coating the magnetic layer with a thin film suited for particular purposes is also utilized in modern practice. The conventional methods have, however, proved unsatisfactory because the lubricants are difficult to mix with or apply on the magnetic coats uniformly, and the lubricating and other effects attained are short-lived, decreasing with use. Further, an increased film thickness leads to an output drop due to spacing loss. Forming a thin, durable coat capable of achieving the desired effects has, therefore, been extremely difficult.
In view of the foregoing disadvantages of the prior art, the present invention has for its object the provision of a magnetic recording medium in general, particularly a high density recording medium, having a novel thin-film top layer which possesses low friction, running stability, durability, corrosion resistance, etc. so as to impart the medium with such improved surface properties.
To this end, it has now been found that a thin film of a polymer having the siloxane linkage exhibits outstandingly advantageous effects. It has also been found that, while it may be formed by heat polymerization or the like, the film achieves even better effects when it is deposited by plasma polymerization on the magnetic layer of a magnetic recording medium. The plasma polymerization produces a film which is as thin as 5 to 1000 .ANG. and yet provides a protective effect and brings the abovementioned improvements in surface properties of the recording medium. Thus any output drop of the medium because of spacing loss is precluded, to great advantage. Since plasma polymerization is a gas-phase reaction, the reaction gas readily gains entrance into minute recesses on the surface of the magnetic layer. Examination under an electron microscope reveals that very small pinholes on the surface are filled up with the gas. This phenomenon appears to play an important role in improving the corrosion-resistant effect of the medium. The plasma polymerization, which makes possible high-speed, continuous film formation, can be easily incorporated into the process for manufacturing the magnetic recording medium, without impairing its productivity.
The thin film of a polymer having the siloxane linkage remarkably improves the surface properties of the medium without in any way sacrificing its magnetic or electric properties or its recording density characteristic. This is a markedly significant advantage of the film over those of the prior art.